This document summarizes research on the optical and structural properties of tenorite (CuO) nanopowders doped with silicon (Si) and zirconium (Zr). Nanopowders were synthesized using a sol-gel method and calcined at temperatures from 400-700°C. X-ray diffraction analysis showed the samples crystallized in a tenorite structure and doping affected crystallinity and particle size. Optical absorption spectroscopy indicated undoped CuO has a direct bandgap of 1.78eV, while doped CuO (15% Si, 15% Zr) has a wider bandgap of 3.75-3.95eV. Scanning electron microscopy and other characterization techniques

1. 1 23
Optical and Quantum Electronics
ISSN 0306-8919
Volume 47
Number 3
Opt Quant Electron (2015) 47:633-642
DOI 10.1007/s11082-014-9940-0
Optical and structural properties of tenorite
nanopowders doped by Si and Zr
Nasrollah Najibi Ilkhechi, Behzad
Koozegar-Kaleji & Fallah Dousi

2. 1 23
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3. Opt Quant Electron (2015) 47:633–642
DOI 10.1007/s11082-014-9940-0
Optical and structural properties of tenorite
nanopowders doped by Si and Zr
Nasrollah Najibi Ilkhechi · Behzad Koozegar-Kaleji ·
Fallah Dousi
Received: 19 March 2014 / Accepted: 25 April 2014 / Published online: 15 May 2014
© Springer Science+Business Media New York 2014
Abstract In this study preparation of Si and Zr doped CuO nanopowders have been inves-
tigated. The effects of Si and Zr doping and heat treatment temperature on the structural and
optical properties of Nanopowders have been studied by X-ray diffraction (XRD), Scanning
electron microscopy (SEM-EDX), UV-Vis absorption and FTIR spectroscopy. Nanopow-
ders were obtained by sol gel method under room conditions (temperature, 25–32◦C) and
were subsequently calcined at different temperatures (400–700◦C). XRD results suggest
that adding impurities has a significant effect on the crystallinity, and particle size of CuO.
The patterns showed that CuO nanopowders calcined at different temperature were Tenorite
structure. The optical absorption spectrum indicates that the CuO nanoparticles have a direct
band gap of 1.78eV. But optical band gap of the doped CuO (15% Si and 15% Zr) was found
to be 3.75–3.95eV.
Keywords CuO nanopowders · Band gap energy · Si and Zr dopant
1 Introduction
Copper oxide-based materials have been widely investigated due to their potential applica-
tions in many fields. Two common forms of copper oxide are cuprous oxide or cuprite (Cu2O)
and cupric oxide or tenorite (CuO) Alkoy and Kelly (2005). Cupric oxide (CuO, tenorite)
is a monoclinic p-type semiconductor with a narrow band gap of 1.2–1.5 eV at room tem-
perature with lattice parameter a = 4.6837, b = 3.4226, c = 5.1288 and β = 99.54 Å
N. N. Ilkhechi (B) · B. Koozegar-Kaleji
Department of Materials Engineering, Faculty of Engineering, Malayer University,
P.O.Box 65719-95863, Malayer, Iran
e-mail: nasernajibi@gmail.com
B. Koozegar-Kaleji
e-mail: b.kaleji@malayeru.ac.ir
F. Dousi
Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
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4. 634 N. N. Ilkhechi et al.
Armelao et al. (2003), whereas cuprous oxide (Cu2O, cuprite) is a cubic (a = 4.253Å) p-
type semiconductor with a direct band gap of 2.0 eV (Armelao et al. 2003; Colon et al. 2006).
Copper oxide, as one of the relatively few metal oxides tend to be p-type, has been widely
exploited for diverse applications such as heterogeneous catalysts, gas sensors, lithium ion
electrode materials, and field emission (FE) emitter (Reitz and Solomon 1998; Switzer et al.
2003; Chowdhuri et al. 2004; Gao et al. 2004; Poizot et al. 2000; Hsieh et al. 2003; Chen et
al. 2003). The nanoparticles, plates, and nanowires of CuO were also reported to sense NO2,
H2S, and CO Cruccolini et al. (2004), Li et al. (2008) Up to now, the reports on the preparation
and characterization of nanocrystalline CuO are comparatively few to some other transition
metal oxides such as zinc oxide (ZnO), titanium dioxide (TiO2), tin dioxide (SnO2) and
iron oxide (Fe2O3), which makes this material an interesting candidate to investigate. Some
methods for the preparation of nanocrystalline CuO have been reported such as the quick
precipitation Zhu et al. (2004), thermal decomposition Kim et al. (2006), chemical reduction
Athawale and Katre (2005), vapor deposition Ponce and Klabunde (2005), electrochemical
Zhang and Wang (2012), microwave irradiation Zhu et al. (2007). sonochemical method
Kumar et al. (2000), sol–gel technique Eliseev et al. (2000), one-step solid state reaction
method at room temperature Xu et al. (2000). Different morphologies have been synthesized
by these methods, such as nanosphers Zhang et al. (2006), nanorods and nanowires Shende
et al. (2008), nanodendrits Li et al. (2004), and nanoflowers Yang et al. (2007). Mohanan
and Brock (2003) have studied copper oxide silica aerogel composites by varying pH values,
copper precursor salts, and treatment temperatures.
They found that based-catalyzed gels underwent a gradual change from bonded Cu2+ to
segregated CuO at different heating conditions. Parler and Ritter (2001). observed silicon-
oxygen-metal bond formation during both synthesis and drying stages at low temperatures
with relative high copper concentrations. Dutta et al. (2003) have studied sol–gel nanocom-
posites containing copper and their gas sensing properties. Also, the selective catalytic reduc-
tions of oxide-supported copper have reported in ref Sullivan and Doherty (2005). ZrO2
presents special characteristics suchas high fracture toughness, ionic conductivity, and sta-
bility evenunder reducing conditions. Moreover, the possession of both amphoteric and redox
functions makes it appealing as a more suitable carrier for a number of catalytic applications
Yamaguchi (1994). As a result, the use of zirconia other than silica or alumina as promoter or
more frequently a support material has attracted considerable interest in recent years Vrinat
and Hamon (1994). The present work reports the preparation and optical characterization
of pure and doped CuO nanopowders using the sol-gel method and its characterization by
using XRD, SEM, EDAX, UV-Vis and FTIR spectroscopic methods. EDX analysis shows
the excellent oxide formation, where in dopant ions are present in the host crystal lattice. The
FTIR analysis shows the stretching vibrations and the oxide formations. We have measured
the energy gap of the different temperature of Si and Zr doped CuO samples from the UV-VIS
absorption spectra and it shows the increase in band gap upon Si and Zr doping as expected.
2 Experimental procedures
2.1 Preparation of the nanopowders
The preparation of precursor solution for Si and Zr doped CuO nanopowders is described as
follows: CuO (C), SiO2(S) and ZrO2(Z) sols were prepared, separately. For the preparation
of CuO sol (Cu(NO3)2.3H2O., Merck) was selected as Copper oxide source. First, 10 ml
distilled water and 4 ml HCL were mixed, and then 1g Cu(NO3)2.3H2O was added to the
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5. Optical and structural properties 635
mixture at the ambient temperature (25◦C). The solution was continuously stirred for 60
min. Solution was aged for 24 h in order to complete all reactions. In order to prepare
SiO2 sol and ZrO2 sol, tetraethoxysilane (Si(OC2H5)4, Aldrich) and zirconyl nitrate hydrate
(ZrO(NO3).2H2O, Aldrich) were dissolved in EtOH with molar ratio of ZrO(NO3).2H2O:
EtOH = 1:20 and Si(OC2H5)4:EtOH = 2:13 at ambient temperature with continuous stirring.
Solutions were aged for 24 h in order to complete all reactions. Then, mixtures of CuO (C),
SiO2 (S), and ZrO2(Z) were made with mol ratios of Si and Zr (C-15%S-15%Z (CSZ)) at the
ambient temperature. The formed gel was dried at 100◦C for 60 min. Finally, the prepared
samples were calcined at desired temperatures (400, 500, 600,700◦C) for 2 h.
2.2 Characterization methods
XRD pattern and phase identification of samples were recorded using X-ray diffraction
analysis (Philips, MPD-XPERT, λ:Cu Kα = 0.154nm). The samples were scanned in the
2θ range of 20–70◦. The average crystallite size of nanopowders (D) was determined from
the XRD patterns, according to the Scherrer equation Vrinat and Hamon (1994)
D = kλ/β cos θ (1)
where k is a constant (shape factor, about 0.9), λ the X-ray wavelength (0.154 nm), β the
full width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle.
The values of β and θ of monoclinic (CuO) phase were taken from (−111) diffraction lines.
Morphology of the nanopowders was observed using scanning electron microscopy (SEM,
XL30 Series) with an accelerating voltage of 10–15 KV, FTIR absorption spectra were
measured over the range of 4,000–500cm−1 at room temperature.
2.3 Band gap energy measurement
The proper amounts of mentioned dispersant (HNO3) was added to 50 ml distilled water
followed by the addition of 0.01g of samples calcined at different temperature for CuO and
C-15%S-15%Z. pH of suspension was adjusted to a desired value, then the suspension was
stirred for 30 min using a magnetic stirrer and subjected to a subsequent treatment in an
ultrasonic bath for 60 min. The specimens were stirred again for 30 min using a magnetic
stirrer. Moreover, the dispersion stability of doped and pure CuO aqueous suspension was
evaluated by the absorbance of suspension using a model mini1240 Shimadzu UV-Visible
spectrometer.
3 Results and discussion
3.1 X-ray diffraction studies of the nanopowders
Figure 1 shows the XRD patterns of the samples without and with 15%Si (S), 10%Zr (Z)
dopant heat treated at different temperature (400, 500, 600, 700◦C) for 2h. According to
the XRD patterns, the pure CuO (C) and co-doped were crystallized in monoclinic (JCPDS:
No. 001–1117) phase and no signs of metal or oxide phases of zirconium or silicon were
detected. By comparing the relative intensity of the diffraction peaks, it can be seen that the
intensity of (−111) plane decreased and the peak position (2θ) is decreased with doping. A
small shift of diffraction peaks towards the lower angle and the decrease of peak intensity by
doping indicates that cations are doped into CuO crystal lattice successfully. The calculated
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6. 636 N. N. Ilkhechi et al.
Fig. 1 XRD spectra of the pure and doped CuO at different tempratures
Table 1 The characteristic of Si
and Zr co-doped CuO
nanopowder thermal treatment at
different temperature
Sample Crystallite
size (nm)
S (m2/g) Band gap
energy (eV)
CuO-400◦C 20.97 46.90 1.78
CSZ-400◦C 20.09 48.96 3.75
CSZ-500◦C 22.94 41.70 3.78
CSZ-600◦C 28.18 34.90 3.83
CSZ-700◦C 29.32 33.54 3.95
crystallite sizes of monoclinic, calculated by scherrer formula, are reported in Table 1. The
crystallite size of doped CuO decreased at 400◦C. It is clear from Table 1 that the crystallite
size slightly decreases from 20.97 to 20.09nm by the addition of Si, Zr dopant whereas
a remarkable increase is observed from 20.97 to 29.32nm when calcination temperature
increased to 700◦C. This is due to the fact that crystallite size decreased with dopant and
agglomeration.
But The increase in crystallite size for doped nanoparticles with increase temperature can
be attributed to the atomic diffusion. From an atomic perspective, diffusion is just the step
wise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials
are in constant motion, rapidly changing positions. For an atom to make such a move the
atom must have sufficient energy to break bonds with its neighbor atoms and then cause
some lattice distortion during the displacement. As the temperature increases the atoms gain
sufficient energy for diffusive motion and thereby increasing the crystallite size Callister
and Rethwisch (2007). It can be seen from XRD spectra that there is a clear shift in peak
positionsofdopedCuOnanoparticlestowardslowerangles,whichindicatesaslightdistortion
in the symmetry of the system due to the creation of defects and vacancies in the system.
Also the Zr4+ radius (0.725Å) is slightly bigger than Cu2+radius (0.585Å) but Si4+ radius
(0.555) is smaller than Cu2+ and both factors could led to slight induced stress in CuO
lattice.
Also, The diffraction peaks at 2θ angle 53 61, 58 18, 61 67, 66 02 and 6799◦ correspond
to diffraction from planes (020), (202), (1−13), (022) and (1 1 3), respectively, of CuO at
high temperature decreased or removed. Based on the data in Table 1, depending on the
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7. Optical and structural properties 637
calcination temperature surface area of the co-doped CuO increased or decreased. Surface
areas increased with increased temperature up to 500◦C and there was a significant increase
of specific surface area from 46.90 to 61.70 m2/g. Table 1 shows that after calcination of the
doped sample at 500–700◦C, a minimum surface area of 33.54 m2/g was measured, which
corresponds with a decrease of 28.5 %. It is common that the surface area decreases with the
elevating temperature owing to the degree of crystallinity. In our cases, the sample calcined
at 700◦C show a higher crystalline structure, leading to a decrease in surface area.
3.2 FTIR analysis of pure and doped CuO nanoparticles
Figure 2 shows FTIR spectra of pure and doped CuO nanoparticles. Infrared studies were
carried out in order to ascertain the purity and nature of the metal nanoparticles. Metal
oxides generally give absorption bands in fingerprint region below 1000 cm−1arising from
inter-atomic vibrations.
The infrared spectra (Fig. 2) of pure and doped CuO exhibited the following bands:
(i) 2,853.20 and 2,924.25cm−1due to band C2H5–O vibrations Zeng et al. (2010).
(ii) 3,404 cm−1 due to inter molecular structure and the O–H band but it could be related
also copper hydroxide presence Sakavanti et al. (2008).
(iii) Weak bands around 1,384cm−1 corresponded to the C–H vibration, indicating few
surfactants absorbed on the surface of CuO samples Liu et al. (2012).
(iv) 524, 472, 469, 471 and 472cm−1which can be attributed to the vibrations of Cu–O Daia
et al. (2007).
(v) Band around 800 cm−1 corresponds to Si–O Okamoto et al. (1997) bending vibration
where the oxygen move at the right angle to the Si–Si lines in the Si–O–Si plane
Okamoto et al. (1997), Morterra et al. (1998).
(vi) Band around 1,054cm−1 due to Si–O–Cu or Si–O–Si bending modes Queeney et al.
(2004).
(vii) Band around 914cm−1 due to N–O stretching vibration Chakradhar et al. (2003).
Based on the data in Fig. 2 At increased temperature the intensity of both hydroxyl bands
decreased, preferentially that of terminal O–H. The decreasing intensity of the hydroxyl
bands is attributed to the formation of metal oxygen band.
Fig. 2 FTIR spectra of pure and doped CuO nanoparticles calcined at different temperatures
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8. 638 N. N. Ilkhechi et al.
Fig. 3 Tauc plots of pure and doped CuO nanoparticles at different temperatures
3.3 Optical evaluation
To estimate the value of the direct band gap of CuO nanoparticles from the absorption spectra
we used the Tauc relation given below Daia et al. (2007).
(αhυ)1/n
= A(hυ − Eg) (2)
where α is absorption coefficient, A a constant (independent from ν) and n the exponent that
depends on the quantum selection rules for the particular material. A straight line is obtained
when (αhν)2 is plotted against photon energy (hν), which indicates that the absorption edge
is due to a direct allowed transition (n = 1 for direct allowed transition).
The intercept of the straight line on hν axis corresponds to the optical band gap (Eg) and its
values determined for CuO nanopowders are shown in Fig. 3 which showed that the blue shift
in the direct band edge as the Si and Zr doped CuO, Such a blue shift has also been reported
in the literature for CuO quantum dots Borgohain and Mahamuni (2002) where the blue shift
has been attributed to the quantum confinement effects of nanoparticles Chakradhar et al.
(2003). The characteristic size known as the exciton Bohr radius, below which one observes a
fundamental shift in electronic and optical properties as a function of size, has been reported
to be in the range of 6.6–28.7nm for CuO Borgohain and Mahamuni (2002).
It can be seen from Tauc plots (Fig. 3) that band gap of pure CuO nanoparticles is 1.78eV.
Also the values of band gap calculated from Tauc plots were found to be 3.73, 3.62, 3.85 and
3.92eV for doped CuO at temperatures 400, 500, 600 and 700◦C respectively. Furthermore,
the band gap showed a significant decrease for the sample calcined at 500◦C, compared
with the graph of photocatalysts prepared at 600 and 700, although the crystallite size was
increased and the band gap energy decreased. Table 1 shows the variation of band gap and
crystallite size with temperatures
3.4 SEM and EDX analysis of pure and doped CuO nanopowders
The SEM images of pure and doped CuO nanopowders calcined at different temperature are
shown in Fig. 4. It can be clearly seen that the microstructures of the powders are strongly
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9. Optical and structural properties 639
Fig. 4 SEM images of pure and doped CuO calcined at different temperatures a CuO-400◦C b CSZ-400◦C
c CSZ-500◦C d CSZ-600◦C e CSZ-700◦C
affected by doped and calcinations temperature. The image of pure CuO powders calcined
at 400◦C as shown in Fig. 4a is in irregular structure comprising flakes. It can be seen from
Fig. 4b that doped CuO have slightly lower particles size as compared to pure CuO, at this
stage, the size distribution range of doped CuO particles was approximately 200–400 nm with
increasingcalcinationstemperatureupto500◦C(Fig.4c),alltheflakeparticleswerefractured
in to the smaller particle sizes and aggregated packing of CuO nanoparticles was formed.
The agglomeration of particles is usually explained as a common way to minimize their
surface free energy, with increase in grain growth. After calcinations temperature at 600◦C
(Fig. 4d) activation, some large platelet or flake shape were formed in the sample and uniform
distribution of particles was achieved. In Fig. 4e, sample contains only agglomerates in the
form of nanoflowers, which are indeed very similar to numerous cauliflower-like structures.
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10. 640 N. N. Ilkhechi et al.
Fig. 5 EDX of Si and Zr co doped CuO at temperature 400◦C
The mean size of the nanoflowers is of about 400–700nm. The nano flowers are not perfectly
spherical and exhibit the petals are packed dense. The EDAX spectra of Si and Zr doped
nanoparticles are shown in Fig. 5. This shows that Si4+ and Zr4+has entered in the crystal
matrix of CuO. It is expected that the Si4+ and Zr4+ ions will be replacing the O2− ions
instead of occupying the interstitials. The doping levels and the bonding characteristics are
determined by EDAX spectrum.
4 Conclusions
This study focused on the effects of calcination temperature and Si and Zr dopants on mor-
phology, crystallite size, and band gap energy of Tenorite nanopowders. The nano- composite
particles were prepared from precursor solutions via sol–gel method and calcinations at a
temperature range of 400–700◦C. Crystalline monoclinic single phase was found at different
calcination temperature for all samples. Crystallite size of doped CuO tends to decrease at
calcination temperatures 400–500◦C then increased at 600– 700◦C. Doping Si and Zr in CuO
was effective on band gap energy of the nanocomposite powders. Band gap energy, greatly
influenced by its crystallinity, grain size, surface areas, and dopant. The optical absorption
band gap of pure and doped CuO nanoparticles is determined to be 1.78 and 3.62–3.95eV
respectivily. SEM results show that the cauliflower-like for doped CuO nanostructure at
700◦C. The incorporation of Si4+ and Zr4+ in the matrix was confirmed by the absorption
spectra.
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